The invention provides a preparation method of carbon quantum dots and belongs to the technical field of preparation of nano luminescent materials. The preparation method comprises the following steps: weighing forestry and agricultural residues, kitchen garbage and mixture of one or more of vegetables, fruits, grains, flowers and Chinese herbal medicines in a ratio of parts by weight of 1:(1-100):(0.5-100) as raw materials; pulverizing the weighed raw materials, sieving the pulverized raw materials, performing uniform mixing, and adding organic acid or strong base or weak base to the treatedmixed liquid; and performing ultrasonic treatment, hydrothermal treatment and centrifugal treatment to obtain a solution, putting the obtained solution after centrifugal treatment into a dialysis bag,and putting the dialysis bag in a beaker filled with distilled water to be subjected to dialysis treatment so as to obtain the carbon quantum dots. The yield of the carbon quantum dots provided by the invention is 50-90%, and the sizes of the prepared carbon quantum dots are 1-2nm; and compared with the existing preparation method of the carbon quantum dots, the preparation method provided by theinvention has the advantages that the preparation process is simple, the raw materials are easy to obtain, and the preparation method is environmentally friendly and harmless.